Ride Along with Rhea

Animation made from raw Cassini image data acquired April 25, 2011


Assembled from 29 raw images taken by the Cassini orbiter on Monday, April 25, this animation brings us along an orbital ride with Rhea as it crosses Saturn’s nighttime face, the planet’s shadow cast across the ringplane. Sister moons Dione and Tethys travel the opposite lane in the background, eventually appearing to sink into Saturn’s atmosphere.

Rhea's heavily cratered surface, imaged by Cassini on October 2010. NASA/JPL/SSI

The exposure varies slightly from frame to frame due to the fact that they are not all taken with the same color channel filter.

Rhea (1,528 kilometers, or 949 miles, wide), Dione (1,123 kilometers, or 698 miles wide) and Tethys (1,066 kilometers, or 662 miles wide) are all very similar in composition and appearance. The moons are composed mostly of water ice and rock, each covered in craters of all sizes and crisscrossed by gouges, scarps and chasms. All three are tidally locked with Saturn, showing the same face to their parent planet in the same way that the Moon does with Earth.

The Cassini spacecraft was 2,227,878 km (1,384,339 miles) from Rhea when the images were taken.

(The original images have not been validated or calibrated. Validated/calibrated images will be archived with the NASA Planetary Data System in 2012.)

Image credit: NASA / JPL / Space Science Institute. Animation by Jason Major.

Stunning New Images From Cassini’s Close Flyby of Rhea

Rhea, Saturn's rings and some sister moons. Credit: NASA/JPL/Space Science Institute

Jia-Rui C. Cook from the Cassini team sent out an alert that raw images from Cassini’s closest flyby of Saturn’s moon Rhea have begun streaming to Cassini’s raw image page, and they are well worth a look. At closest approach, Cassini came within about 69 kilometers (43 miles) of Rhea’s surface on Jan. 11. But there’s also some interesting group photos from within the Saturn System. One of the best is this image, above. How many moons can you find? I probably wouldn’t have seen them all but Emily Lakdawalla at the Planetary Blog spied five moons and the rings in this one wide-angle shot. The large moon is Rhea; above Rhea and just below the rings, is Dione; above and to the left of Rhea is Tethys. Then there are two tiny moons: squint hard to see Prometheus as tiny lump on the rings to the left of Dione, and Epimetheus is hovering between Tethys and Rhea. See some more, including closeups of Rhea and Saturn’s storm, below.

Continue reading “Stunning New Images From Cassini’s Close Flyby of Rhea”

New Images Indicate Tectonic Activity on Rhea

Hemispheric color differences on Saturn's moon Rhea are apparent in this false-color view from NASA's Cassini spacecraft. This image shows the side of the moon that always faces the planet. Image Credit: NASA/JPL/SSI


Saturn’s second largest moon Rhea has gotten a couple of close-up looks by the Cassini spacecraft which show dramatic views of fractures cutting through craters on the moon’s surface. The new images reveal a history of tectonic rumbling, scientists say. The images are among the highest-resolution views ever obtained of Rhea, including a 3-D look at a tectonically fractured region showing cracks as deep as 4 kilometers (2.5 miles).

“These recent, high-resolution Cassini images help us put Saturn’s moon in the context of the moons’ geological family tree,” said Paul Helfenstein, Cassini imaging team associate, based at Cornell University, Ithaca, N.Y. “Since NASA’s Voyager mission visited Saturn, scientists have thought of Rhea and Dione as close cousins, with some differences in size and density. The new images show us they’re more like fraternal twins, where the resemblance is more than skin deep. This probably comes from their nearness to each other in orbit.”

Cassini made to two close passes of Rhea on Nov. 21, 2009 and March 2, 2010, and the flybys were designed in part to search for a ring thought to encircle the moon, the existence of which has now been ruled out. During the March flyby, Cassini made its closest- approach to Rhea’s surface so far, swooping within 100 kilometers (62 miles) of the moon.

Icy fractures on Saturn's moon Rhea reflect sunlight brightly in this high-resolution mosaic created from images captured by NASA's Cassini spacecraft during its March 2, 2010, flyby. This flyby was the closest flyby of Rhea up to then. Image Credit: NASA/JPL/SSI

These unique views are among the best ever obtained of the side of Rhea that always faces away from Saturn. Other views show a web of bright, “wispy” fractures resembling some that were first spotted on another part of Rhea by the two Voyager spacecraft in 1980 and 1981. These images are helping to answer questions scientists have had about Rhea since the Voyager mission.

At that time, scientists thought the wispy markings on the trailing hemispheres – the sides of moons that face backward in the orbit around a planet – of Rhea and the neighboring moon Dione were possible cryovolcanic deposits, or the residue of icy material erupting. The low resolution of Voyager images prevented a closer inspection of these regions. Since July 2004, Cassini’s imaging cameras have captured pictures the trailing hemispheres of both satellites several times at much higher resolution. The images have shown that the wispy markings are actually exposures of bright ice along the steep walls of long scarps, or lines of cliffs, which indicate tectonic activity produced the features rather than cryovolcanism.

Wispy fractures cut through cratered terrain on Saturn's moon Rhea in this high resolution, 3-D image from NASA's Cassini spacecraft. The image shows a level of detail not seen previously. Image Credit: NASA/JPL/SSI

Scientists combined images of the trailing hemisphere taken about one hour apart to create a 3-D image revealing a set of closely spaced troughs that sometimes look linear and sometimes look sinuous. The 3-D image also shows uplifted blocks interspersed through the terrain that cut through older, densely cratered plains. While the densely cratered plains imply that Rhea has not experienced much internal activity since its early history that would have repaved the moon, these imaging data suggest that some regions have ruptured in response to tectonic stress more recently. Troughs and other fault topography cut through the two largest craters in the scene, which are not as scarred with smaller craters, indicating that these craters are comparatively young. In some places, material has moved downslope along the scarps and accumulated on the flatter floors.

A mosaic of the March flyby images shows bright, icy fractures cutting across the surface of the moon, sometimes at right angles to each other. A false-color view of the entire disk of the moon’s Saturn-facing side reveals a slightly bluer area, likely related to different surface compositions or to different sizes and fine-scale textures of the grains making up the moon’s icy soil.

This global digital map of Saturn's moon Rhea was created using data obtained by NASA's Cassini and Voyager spacecraft. Image Credit: NASA/JPL/SSI

The new images have also helped to enhance maps of Rhea, including the first cartographic atlas of features on the moon complete with names approved by the International Astronomical Union. Cassini will continue to chart the terrain of this and other Saturnian moons with ever-improving resolution, especially for terrain at high northern latitudes, until 2017.

An upcoming flyby should provide even more details about Rhea.

“The 11th of January 2011 will be especially exciting, when Cassini flies just 76 kilometers [47 miles] above the surface of Rhea,” said Thomas Roatsch, a Cassini imaging team scientist based at the German Aerospace Center Institute of Planetary Research in Berlin. “These will be by far the best images we’ve ever had of Rhea’s surface – details down to just a few meters will become recognizable.”

For more images and for higher resolution versions of the ones seen here, see the CICLOPS website, or NASA’s Cassini website.

Source: JPL

Tenuous Oxygen Atmosphere Found Around Saturn’s Moon Rhea

The 2 March 2010 Rhea flyby trajectory and simulated oxygen atmosphere distribution. Inset: Predicted oxygen density (yellow) compared to the INMS measurement (white) during the flyby. Image © Science/AAAS


A few years ago, astronomers thought they found wispy rings around Saturn’s moon Rhea. Although the possibility of rings around this icy moon was later nixed, astronomers knew there was still something around Rhea that was causing a strange, symmetrical structure in the charged-particle environment around Saturn’s second-largest moon. Now, new observations have shown something else around Rhea that was completely unexpected: an oxygen atmosphere. In March of this year, the Cassini spacecraft made a close flyby of Rhea and recorded data showing a thin atmosphere made up of oxygen and carbon dioxide.

The source of the oxygen is not really a surprise: Rhea’s density of 1.233 times that of liquid water suggests that Rhea is three quarters ice and one quarter rock. The moon’s tenuous atmosphere is maintained by the ongoing chemical decomposition of ice water on the moon’s surface by irradiation from Saturn’s magnetosphere.

Oxygen has also recently been detected in the atmospheres of two of Jupiter’s moons, Europa and Ganymede. Since oxygen is a main component of the atmosphere surrounding Saturn’s rings, astronomers think there could be similar atmospheres around other icy moons that orbit inside Saturn’s magnetosphere.

“The new results suggest that active, complex chemistry involving oxygen may be quite common throughout the solar system and even our universe,” said lead author Ben Teolis, a Cassini team scientist based at Southwest Research Institute in San Antonio. “Such chemistry could be a prerequisite for life. All evidence from Cassini indicates that Rhea is too cold and devoid of the liquid water necessary for life as we know it.”

Of course, there’s always the possibility of life as we don’t know it.

And, there must be some sort of organics on the moon – meaning carbon compounds. The source of the carbon dioxide in Rhea’s atmosphere is not yet known, but its presence suggests that radiolysis reactions between oxidants and organics are ongoing at the moon’s surface.

As far as any of these new findings having a relation to the ruled-out hypothesis of rings around Rhea, Teolis told Universe Today there is still much about Rhea’s environment that is yet to determined. “The electron depletion is currently unexplained,” Teolis said in an email. The sharp, symmetrical drop in electrons detected around Rhea was the initial finding behind the ring theory. “Our current thinking is that it may be related to the ionization of the atmosphere, perhaps in conjunction with electrostatic charging of Rhea’s surface, but I do not have a definitive answer at this point. The atmosphere – magnetosphere interaction is a complex problem, and will take some time to sort out. But for the first time at an icy moon, the Cassini findings give us an in situ observational window onto this interaction, understanding of which is still highly theoretical. We’re working on it.”

Rhea, as seen by Cassini. Credit: NASA

This latest data came from Cassini’s ion and neutral mass spectrometer and the Cassini plasma spectrometer during flybys on Nov. 26, 2005, Aug. 30, 2007, and March 2, 2010. The ion and neutral mass spectrometer saw peak densities of oxygen of around 50 billion molecules per cubic meter (1 billion molecules per cubic foot). It detected peak densities of carbon dioxide of around 20 billion molecules per cubic meter (about 600 million molecules per cubic foot).

The plasma spectrometer saw clear signatures of flowing streams of positive and negative ions, with masses that corresponded to ions of oxygen and carbon dioxide.

The scientists said the oxygen appears to rise to an atmosphere when Saturn’s magnetic field rotates over Rhea. Energetic particles trapped in the planet’s magnetic field pepper the moon’s water-ice surface. They cause chemical reactions that decompose the surface and release oxygen.

Releasing oxygen through surface irradiation could help generate conditions favorable for life at an icy body other than Rhea that has liquid water under the surface, Teolis said. If the oxygen and carbon dioxide from the surface could somehow get transported down to a sub-surface ocean, that would provide a much more hospitable environment for more complex compounds and life to form.

The scientists are unsure how the carbon dioxide is released. It could be the result of “dry ice” trapped from the primordial solar nebula, as is the case with comets, or it may be due to similar irradiation processes operating on the organic molecules trapped in the water ice of Rhea. The carbon dioxide could also come from carbon-rich materials deposited by tiny meteors that bombarded Rhea’s surface.

“Rhea is turning out to be much more interesting than we had imagined,” said Linda Spilker, Cassini project scientist at JPL. “The Cassini finding highlights the rich diversity of Saturn’s moons and gives us clues on how they formed and evolved.”

This research appears in the November 25, 2010 issue of Science Express.

Sources: Science, JPL, email exchange with Teolis

Conjoined Moons

Saturn's moons Dione and Rhea appear conjoined in this optical illusion-like image taken by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute


This latest image from the Cassini spacecraft will make you do a double-take! It is an optical illusion, but the two moons appear like conjoined, identical twins! The two moons are fairly close in size, but Dione, the smaller of the two at the top in the image, is actually closer to the spacecraft, making the two look almost identical. And because of the similar albedo, or reflectivity, of the two moons and because of the location of a particularly large crater near the south polar region of Dione, the moon appears blended seamlessly with Rhea. Double your pleasure!

Dione is 1123 kilometers (698 miles) across and Rhea is 1528 kilometers (949 miles) across.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on July 27, 2010.

See more about the image at the CICLOPS website.

Ring Around Rhea? Probably Not

Rhea, taken by the Cassini spacecraft in March, 2010. Credit: NASA/JPL/Space Science Institute


Back in 2005, a suite of six instruments on the Cassini spacecraft detected what was thought to be an extensive debris disk around Saturn’s moon Rhea, and while there was no visible evidence, researchers thought that perhaps there was a diffuse ring around the moon. This would have been the first ring ever found around a moon. New observations, however, have nixed the idea of a ring, but there’s still something around Rhea that is causing a strange, symmetrical structure in the charged-particle environment around Saturn’s second-largest moon.

Researchers announced their findings in 2008 that there was a sharp, symmetrical drop in electrons detected around Rhea. This moon is about 1,500 kilometers (950 miles) in diameter, and scientists began searching for what could have caused the drop. If there were a debris disk around Rhea, it would have had to measure several thousand miles from end to end, and would probably be made of particles that would range from the size of small pebbles to boulders.

Testing the hypothesis, Cassini flew by the moon several times and took 65 images between 2008 and 2009, flying at what would be edge-on to the rings, where the greatest amount of material would be within its line of sight.

Using light angles to their advantage — and if the ring was there – the scientists should have been able to detect micron-sized particles up to boulder size objects.

But they saw nothing.

“There are very strong and interesting and unexplained electromagnetic effects going on around Rhea,” said Matthew Tiscareno from Cornell University, who led the imaging campaign. “But we’re making a pretty strong case that it’s not because of solid material orbiting the moon….For the amount of dust that you need to account for [the earlier] observations, if it were there, we would have seen it.”

While the ring hypothesis has been disproved, there’s still a mystery about the cause of the symmetrical structure in the charged-particles around the moon.

But the Cassini spacecraft and team are up for the challenge.

Source: Cornell University

Latest Wall Art from Cassini

Rhea poses with Saturn's rings; Janus and Prometheus are off in the distance. Credit: NASA/JPL/Space Science Institute. Click for larger version


Oh, wow — what a gorgeous image! Just the latest from our resident artist in space, the Cassini spacecraft. Rhea, saturn’s second largest moon sits in front of the rings, joined by two smaller moons in the background. Rhea (1528 kilometers, 949 miles across) is in the center foreground. Janus (179 kilometers, 111 miles across) can be seen beyond the rings on the right of the image. Prometheus (86 kilometers, 53 miles across) is visible orbiting between the main rings and the thin F ring on the left of the image. Lit terrain seen on Rhea is on the area between that moon’s trailing hemisphere and anti-Saturn side. This view looks toward the northern, sunlit side of the rings from just above the ringplane.

If you like contrast images, there’s a great one below.

Saturn's rings contrast with the blackness of space. Credit: NASA/JPL/Space Science Institute

This image is a beautiful contrast between dark and light. Atlas can be seen just above the center of this Cassini spacecraft image as the moon orbits in the Roche Division between Saturn’s A ring and thin F ring.

Sources: CICLOPS, Cassini

Stunning Cassini Image to Knock Your Socks Off

What a way to start the day! This image is one of the first things I saw online this morning. The moon Rhea hangs like a pendant against Saturn and its rings. Amazingly, this is a raw image straight from Cassini; it has not been calibrated or enhanced in any way. This is art in its purest form and evidence of the phenomenal and enchanting beauty of the Saturn system, as well as confirmation of what an amazing spacecraft Cassini is.

This image was taken on November 08, 2009 and received on Earth November 09, 2009. The camera was pointing toward Rhea at approximately 1,874,061 kilometers away.

Here’s the link to a higher resolution version.

Source page.

Hat tip to @mars_stu on Twitter